This invention relates to lithographic systems and techniques that are used in, for example, integrated circuit manufacturing; and more particularly, in one aspect, to measure, inspect, characterize and/or evaluate optical lithographic equipment, methods, and/or subsystems related thereto (for example, the optical sub-systems and control systems of the equipment as well as photomasks used therewith).
In the fabrication of integrated circuit, lithography is employed to xe2x80x9cprintxe2x80x9d circuit patterns on a wafer (e.g., silicon or GaAs semiconductor substrate). Currently, optical lithography is the predominant form of lithography used in volume integrated circuit manufacturing. Optical lithography typically employs visible or ultraviolet light to expose a given pattern (generally defined by the photomask) on the resist that is disposed on a wafer to be transferred into the substrate through resist development and subsequent process steps, for example, etching, deposition and implantation
In optical lithography, the photomask (or mask), is first written using electron-beam or laser-beam direct-write tools. The mask contains certain patterns and features that are used to create desired circuit patterns on a wafer. The process of fabricating a complete integrated circuit typically requires the use of many masks.
In the field of integrated circuit manufacturing, a common lithographic tool used in projecting an image or pattern formed in a photomask onto a wafer is known as a xe2x80x9cstepperxe2x80x9d or xe2x80x9cscannerxe2x80x9d. With reference to FIG. 1, lithographic equipment 10 (for example, a stepper) may include mirror 12, light source 14 to generate light 16 at, for example, an exposure wavelength xcex0. The lithographic equipment 10 may also include illumination optics 18, projection optics 20, and a chuck 22 upon which a wafer 24 is temporally secured, typically by way of electrostatic or vacuum forces, in a wafer plane. The mask 26 is positioned and optically aligned to project an image of the circuit pattern to be duplicated onto wafer 24. The lithographic equipment 10 may employ a variety of well known stepping, scanning or imaging techniques to produce or replicate the mask pattern on wafer 24.
In general, there are three stages at which the integrity of the lithography process is measured, characterized or inspected. First, the mask is inspected to determine whether the pattern on the mask accurately represents the desired mask design. Second, the optics of the stepper (for example, light source 14, illumination optics 18, and projection optics 20) are measured or characterized to confirm that they are within acceptable guidelines. Third, the pattern xe2x80x9cprintedxe2x80x9d or formed on the wafer or die (discrete pieces of the wafer) is inspected and analyzed to determine or measure the quality of the fabrication process.
The photomasks are typically inspected first by the photomask fabricator before providing them to an integrated circuit manufacturer, and then periodically by the integrated circuit manufacturer, for example, during initial mask qualification and re-qualification. The fabricator and manufacturer tend to use standalone equipment, for example, tools made by KLA-Tencor (e.g., TeraStar series equipment) or Applied Materials (e.g., ARIS-1001 equipment). This standalone equipment, among other things, assesses the accuracy or integrity of the photomask as well as its ability to produce an accurate representation of the circuit design onto the wafer or die, when used in conjunction with appropriate stepper optics and settings. While such inspection equipment may provide an accurate representation of the photomask, it tends to be quite expensive and hence its use tends to be minimized.
Moreover, such inspection equipment often employs optical imaging systems (or sub-systems) that are fundamentally different from that used by the stepper in xe2x80x9cprintingxe2x80x9d the image on the wafer during mass production. For example, such standalone tools may include optical imaging systems that employ wavelengths that are different from optical imaging systems used in the mass production steppers. The response and/or characteristics of photomask may depend upon the wavelength of the light used to measure or detect the mask (via, for example, an aerial image). Indeed, a photomask may exhibit defects in the production stepper environment that may not be detectable in the standalone inspection tool because, for example, detection of certain contaminants depends on wavelength. That is, certain contaminants may present serious issues at the wavelength used during production but may be undetectable at the wavelength used during inspection.
The optics of the stepper are typically characterized by the manufacturer after the stepper is manufactured using grating and wavefront interference methods. The manufacturer may also employ scanning electron microscopy (SEM) techniques to measure the patterns printed, formed or projected on test wafers. In this regard, the manufacturer typically uses photomasks having specifically designed test patterns. In this way, a resist pattern developed on a test wafer may be measured using SEM techniques and compared to a known, predetermined, fixed or expected pattern.
Due primarily to complexity of the inspection techniques, the inspection procedure of the stepper tends to require or consume an extended period of time, often days to complete, and thus represents an expensive procedure for the integrated circuit manufacturer to carry out.
The integrated circuit manufacturer, however, may inspect and evaluate a stepper indirectly, using SEM inspection and analysis of the developed resist image. Here again, due to the extended test time, inspection of the stepper is not performed very often, and, as a result, there are few samples and/or data to form a reliable measure of the stepper.
Conventional techniques to evaluate the final printed circuit pattern on the wafer or die tend to require examining the pattern formed on the wafer using SEM techniques. In this regard, the characterization or verification of the accuracy and quality of the circuit pattern permits an indirect method of characterizing or verifying the mask and stepper (including optics), as well as the interactions between the mask and stepper. Because the final printed circuit pattern on the wafer or die is formed after the resist development and may be after substrate treatment (for example, material etching or deposition), it may be difficult to attribute, discriminate or isolate errors in the final printed circuit pattern to problems associated with the photomask, the stepper, or the resist deposition and/or the developing processes. Moreover, as with inspection of the optics of the stepper, inspecting the final printed circuit pattern on the wafer or die using an SEM tends to offer a limited number of samples upon which to detect, determine, and resolve any processing issues. This process may be labor intensive and presents an extensive inspection and analysis time.
Thus, there is a need for a system and technique to overcome the shortcomings of one, some or all of the conventional systems and techniques. In this regard, there is a need for an improved system and technique to inspect and characterize optical lithographic equipment, including the optical sub-systems, control systems and photomasks, that are used in, for example, integrated circuit manufacturing.
In addition, there is a need for a system and technique of photomask inspection and characterization of in-situ or in a mass production/fabrication environment. In this regard, there is a need for a system and technique to measure, sense, inspect, detect, capture and/or evaluate the aerial image of a photomask in situxe2x80x94that is, in the mass production environment using the lithographic production equipment of that environment. In this way, the errors may be isolated and attributed to a given aspect of the process or system. Indeed, the causes of errors in a final printed circuit pattern may be isolated, characterized and/or measured (in, for example, the photomask, stepper, and/or resist developing process) so that appropriate corrective measures may be determined efficiently, rapidly and in a cost-effective manner. Thus, there is a need for a system and technique that permits errors in the lithographic fabrication process to be attributed or isolated to certain methods or equipment (for example, the photomask or optical sub-system) in order to facilitate appropriate and/or efficient correction of such errors in the final printed circuit pattern and thereby enhance or improve the quality, yield and cost of integrated circuits.
Further, there is a need for an improved lithographic image evaluation technique and system that overcomes one, some or all of the conventional systems and techniques. In this regard, there is a need for a system and technique to more thoroughly, quickly and/or more often evaluate and calibrate lithographic imaging systems, for example, steppers, in an efficient and cost-effective manner. In this way, the quality, yield and cost of integrated circuits may be improved.
There are many inventions described herein. In a first principal aspect, the present invention is an image sensor unit, for use with a highly precise moveable platform. The image sensor unit of this aspect of the invention includes a substrate having a wafer-shaped profile or form factor that may allow automated handling of image sensor unit in the same manner as a product-type wafer. The image sensor unit further includes a sensor array (for example, charge coupled, CMOS or photodiode devices) disposed on the substrate.
The sensor array includes a plurality of sensor cells wherein each sensor cell includes an active area to sense light of a predetermined wavelength that is incident thereon. The sensor array also includes a film, disposed over the active areas of sensor cells and comprised of a material that impedes passage of light of the predetermined wavelength. The film includes a plurality of apertures that are arranged such that at least one aperture overlies an active area of a corresponding sensor cell to expose a portion of the active area and wherein light of the predetermined wavelength is capable of being sensed by the portion of the active area that is exposed by the corresponding aperture.
In one embodiment of this aspect of the invention, the image sensor unit may include a transparent medium, having a predetermined refractive index, disposed on the sensor array. In another embodiment, the image sensor unit may include photon-conversion material disposed over and/or within the sensor array. The photo-conversion material may be disposed between the film and the plurality of sensors.
In another embodiment, the image sensor unit may include communications circuitry disposed on the substrate. The communications circuitry may employ wired, wireless and/or optical techniques. In one embodiment, the communications circuitry outputs data from the sensor array, using wired and/or wireless techniques, during collection of image data by the sensor array.
In another embodiment, the image sensor unit may include at least one battery, disposed on the wafer-shaped substrate or within a cavity in the wafer-shaped substrate. The battery may be rechargeable and may provide electrical power to the sensor array and/or the communications circuitry.
In another embodiment, the image sensor unit may also include data storage circuitry and data compression circuitry. In this embodiment, the data storage circuitry is coupled to the sensor array to receive and store the data from the sensor array. The data compression circuitry is coupled to the data storage circuitry to compress the data.
In another principal aspect, the present invention is an image sensor unit, for use with a highly precise moveable platform, which includes a wafer-shaped substrate and a sensor array, integrated into the substrate. The sensor array includes a plurality of sensor cells (for example, charge coupled devices, CMOS devices or photodiodes) wherein each sensor cell includes an active area to sense light of a predetermined wavelength that is incident thereon. The sensor array also includes a film, disposed over the plurality of active areas of the sensor cells and comprised of a material that impedes passage of light of the predetermined wavelength. The film includes a plurality of apertures that are arranged such that an aperture of the plurality of apertures overlies an active area of a corresponding sensor cell to expose a portion of the active area. In this way, light of the predetermined wavelength is capable of being sensed by the portion of the active area that is exposed by the corresponding aperture.
In one embodiment of this aspect of the present invention, the image sensor unit may include communications circuitry disposed on the substrate. The communications circuitry may employ wired, wireless and/or optical techniques. In one embodiment, the communications circuitry outputs data from the sensor array, using wireless techniques, during collection of image data by the sensor array.
In another embodiment, the image sensor unit may include at least one battery, disposed on the wafer-shaped substrate or within a cavity in the wafer-shaped substrate. The battery may be rechargeable.
In yet another embodiment, the image sensor unit may also include data storage circuitry and data compression circuitry. In this embodiment, the data storage circuitry is coupled to the sensor array to receive and store the data from the sensor array. The data compression circuitry is coupled to the data storage circuitry to compress the data.
The image sensor unit may include photon-conversion material disposed over and/or within the sensor array. In another embodiment, the photo-conversion material is disposed between the film and the plurality of sensors.
In yet another principal aspect, the present invention is a system to collect image data which is representative of an aerial image of a mask (for example, a product-type or test mask) that is projected on a wafer plane. The system includes an optical system to produce the image of the mask on the wafer plane, a moveable platform and an image sensor unit, disposed on the moveable platform, to collect image data which is representative of the aerial image of the mask.
The image sensor unit includes a wafer-shaped substrate and a sensor array. The sensor array is disposed on or in the wafer-shaped substrate, such that when position on the moveable platform, the sensor array is disposed in the wafer plane.
The sensor array includes a plurality of sensor cells wherein each sensor cell includes an active area to sense light of a predetermined wavelength that is incident thereon. The sensor array further includes a film, disposed over the active areas of the sensor cells. The film is comprised of a material that impedes passage of light of the predetermined wavelength and includes a plurality of apertures which are arranged such that an aperture of the plurality of apertures overlies a corresponding active area of a corresponding sensor cell to expose a portion of the active area. In this way, the light of the predetermined wavelength is capable of being sensed by the portion of the active area that is exposed by the corresponding aperture.
In one embodiment of this aspect of the present invention, the image sensor unit may include communications circuitry disposed on the substrate. The communications circuitry may employ wired, wireless and/or optical techniques. In one embodiment, the communications circuitry outputs data from the sensor array, using wired and/or wireless techniques, during collection of image data by the sensor array.
In another embodiment, the image sensor unit may include a data processing unit and/or at least one battery (for example, a rechargeable-type), disposed on, or within a cavity in, the wafer-shaped substrate, to provide electrical power to the sensor array and/or the communications circuitry. The data processing unit may be configured to receive the image data which is representative of the aerial image.
In one embodiment, the moveable platform may move in first and second directions to a plurality of discrete locations wherein at each discrete location, the sensor cells sample the light incident on the exposed portion of the active area. The data processing unit may use the data to generate the aerial image.
The distance between the plurality of discrete locations in the first direction may be less than or equal to the width of the apertures. Further, the distance between the plurality of discrete locations in the second direction may be less than or equal to the width of the apertures. In one embodiment, the processing unit interleaves the image data to generate the aerial image.
In one embodiment, the image sensor unit collects data which is representative of the aerial image in a raster-type manner. In another embodiment, the image sensor unit collects image data which is representative of the aerial image in a vector-type manner.
In another aspect, the present invention is an image sensor unit that may be employed to collect image data which is representative of an aerial image of a mask (for example, a product-type mask) that is projected on a wafer plane by a lithographic unit. The image sensor unit of this aspect of the invention includes a sensor array which is disposed in the moveable platform of the lithographic unit and capable of being located in the wafer plane. The sensor array (for example, a charge coupled, CMOS or photodiode device) includes a plurality of sensor cells wherein each sensor cell includes an active area to sense light of a predetermined wavelength that is incident thereon. The sensor array also includes a film, disposed over the active areas of the plurality of sensor cells and comprised of a material that impedes passage of light of the predetermined wavelength. The film includes a plurality of apertures which are arranged such that an aperture of the plurality of apertures overlies a corresponding active area of a corresponding sensor cell to expose a portion of the active area so that light of the predetermined wavelength is capable of being sensed by the portion of the active area that is exposed by the corresponding aperture.
In one embodiment, the sensor array is capable of being moved between a plurality of discrete locations in first and second directions while disposed on the moveable platform. The sensor cells sample the light incident on the exposed portion of the active area at each discrete location. The distance between the plurality of discrete locations in the first direction may be less than or equal to the width of the apertures. Further, the distance between the plurality of discrete locations in the second direction may be less than or equal to the width of the apertures. In one embodiment, the processing unit interleaves the image data to generate the aerial image.